How to produce realistic wood material
The output socket of your normal map node should be plugged in the normal input of the upper Diffuse BSDF. The second Diffuse BSDF is completely unuseful, as the displacement input of the material output node expects a greyscale value (the socket is grey) and not a shader value (green sockets). For a better result you could also modulate a mix of diffuse and glossy shader, both with a reflections texture and the Fresnel effect, which gives you more reflections on the borders of your material (according to the angles of light reflections). At the end of this process you can decide if it's necessary to subdivide your mesh and add some displacement maps, augmenting the poly-count.